A file server is a type of storage server that operates on behalf of one or more clients to store and manage shared files in a set of mass storage devices, such as magnetic or optical storage based disks. The mass storage devices are typically organized as one or more groups of Redundant Array of Independent (or Inexpensive) Disks (RAID). One configuration in; which file servers can be used is a network attached storage (NAS) configuration. In a NAS configuration, a file server can be implemented in the form of an appliance that attaches to a network, such as a local area network (LAN) or a corporate intranet. Examples of such an appliance include any of the NetApp Files products made by Network Appliance, Inc. in Sunnyvale, Calif., or the SpinServer® products formerly made by Spinnaker Networks, Inc., which was acquired by Network Appliance.
A file server can be used to store data redundantly, among other things. One particular type of data redundancy technique is known as data replication, also referred to as “mirroring”. Mirroring involves making a copy of data stored at a primary site by storing an exact duplicate (a mirror image) of the data at a remote secondary site. In the mirroring context, the copy at the primary site is generally referred to as the data source, and the secondary site where the copy is made is referred to as the data destination. Thus, the term mirror image can be used interchangeably with data destination, mirroring data destination, data replication data destination, and so on. The purpose is that, if data is ever lost at the primary site, it can be recovered from the secondary site.
In a simple example of a mirroring configuration, a source file server located at the primary site may be coupled locally to a first set of mass storage devices, to a set of clients through a local area network (LAN), and to a destination file server located at a remote site through a wide area network (WAN) or metropolitan area network (MAN). The destination storage server located at the remote site is coupled locally to a second set of mass storage devices at the secondary site.
Additional safeguards against data loss can be implemented by a file server. One such safeguard is data backup. During a backup operation, data is copied to some backup facility, generally some mass offline storage such as digital tape. There are several standardized backup protocols available, such as the Network Data Management Protocol (NDMP) data backup. There are also various mass offline storage facilities available, such as digital tape libraries.
When a portion of the data being backed up is corrupted, the backup application can crash during execution. This results in no data being backed up and a waste of resources, as the backup may have been executing for hours before the crash. Furthermore, the administrator would then have to find and remedy the corruption by executing some data repair program, such as spiff, that can take hours to locate the corrupt data.